1. Field of the Invention
The present invention relates generally to an apparatus and method for receiving a Digital Multimedia Broadcasting (DMB) service in a mobile station, and in particular, to an apparatus and method for receiving a DMB service using an external antenna in a mobile station.
2. Description of the Related Art
Generally, a digital broadcasting system refers to a system that provides to users advanced services having high video quality and CD-class audio quality, replacing the conventional analog broadcasting. A DMB service is one of the typical digital broadcasting services. The DMB service has been proposed to modulate various multimedia signals such as audio and video signals with digital technology and provide the modulated multimedia signals to fixed, portable, and/or vehicle receivers (or terminals). The DMB service is also referred to as “TV in Hand.”
The DMB service is based on Digital Audio Broadcast (DAB) technology, which is digital radio technology. The DMB service, though it is based on the DAB technology, can not only provide the audio service, but is also a multimedia service capable of transmitting moving images, weather information, news, position information, packet data, and the like. Presently, the DMB service is classified into a terrestrial DMB service and a satellite DMB service.
The satellite DMB service adopts a scheme of allowing a satellite to directly transmit signals to receivers, and providing broadcast services to the receivers via terrestrial repeaters. With reference to FIG. 1, a description will now be made of the satellite DMB service.
FIG. 1 is a network architecture diagram of a broadcasting system for providing the up-to-date satellite DMB service.
Referring to FIG. 1, a satellite DMB broadcasting center 100 on the ground transmits broadcast signals to a DMB satellite 106 through a Ku-band (12 GHz to 13 GHz). The signals transmitted from the satellite DMB broadcasting center 100 to the DMB satellite 106 are transmitted by Time Division Multiplexing (TDM) 102 and Code Division Multiplexing (CDM) 104. The DMB satellite 106 processes the received broadcast signals 102 and 104, and transmits the processed broadcast signals back to a receiving terminal 116 on the ground, or a gap filler 108 that serves as a terrestrial repeater.
The DMB satellite 106, when it directly exchanges signals with the terminal 116 on the ground, processes the received broadcast signals into CDM signals 112 in an S-band (2 to 3 GHz) before transmission. The DMB satellite 106, when it transmits signals to the gap filler 108, converts the received broadcast signals into TDM signals 110 in a Ku-band before transmission. Although only one gap filler 108 is shown in FIG. 1, the number of gap fillers is extendable according to characteristic of service areas. The DMB satellite 106 transmits the broadcast signals to the gap filler 108 to provide extended coverage for the broadcast signals transmitted by the satellite, for example into shadow areas (or blanket areas) like a basement. The gap filler 108 converts the broadcast signals transmitted by the DMB satellite 106 into S-band signals, and transmits the S-band signals with radio signals in its area, i.e. shadow area, thereby providing the DMB service to the terminal 116 located in the shadow area. The gap filler 108 transmits transmission broadcast signals 114 together with its uniquely allocated gap filler identifier (ID), and allows the terminal 116 receiving the broadcast signals 114 to identify the gap filler from which it is receiving the broadcast signals. Similarly, the DMB satellite 106 also transmits the broadcast signals 112 along with a unique satellite ID. As a result, the mobile terminal 116 can identify the gap filler or the DMB satellite from which it is receiving the broadcast signals.
The mobile terminal 116 includes an antenna for receiving the broadcast signals transmitted from the gap filler 108 and/or the DMB satellite 106. Generally, the antenna is mounted within the mobile terminal 116, and is referred to as an internal antenna (or embedded antenna). The internal antenna has good reception performance for the broadcast signals transmitted from the gap filler 108, but has very poor reception performance for the broadcast signals transmitted from the DMB satellite 106. Therefore, the mobile terminal (or DMB receiver) 116 is equipped with an additional external antenna to receive the broadcast signals transmitted from the DMB satellite 106.
There is a possible area where the broadcast signals 112 received directly from the DMB satellite 106 and the broadcast signals 114 received from the gap filler 108 coexist. If the mobile terminal 116 uses the external antenna in this area, the broadcast signals 112 and the broadcast signals 114 may serve as interference with each other, causing a decrease in Signal-to-Noise Ratio (SNR) measured by the mobile terminal 116. The decrease in the SNR deteriorates the reception performance, which in turn can cause pauses or result in an impossibility of broadcast viewing.
A description will now be made of the situation in which the SNR decreases.
Generally, in viewing satellite DMB broadcast, users receive satellite DMB broadcast signals with the external antenna attached to the mobile terminal 116, regardless of the current reception performance. This is because the attachment of the external antenna helps improve the antenna gain, increasing reception sensitivity. However, if the user is now viewing the broadcast at the coverage edge of the gap filler 108, the signals received from the gap filler 108 and the signals received from the DMB satellite 106 may be similar in strength. In this situation, the attachment of the external antenna may considerably deteriorate the reception performance. That is, the SNR may decrease.
This phenomenon occurs because the broadcast signals 112 directly received from the DMB satellite 106 and the broadcast signals 114 received from the gap filler 108 have a linear polarization form. The broadcast signals 112 in a circular polarization form, directly received from the DMB satellite 106, can be normally received via the external antenna, but are rarely received via the internal antenna. In addition, the broadcast signals 114 having a linear polarization form, received from the gap filler 108, can be received via both the internal antenna and the external antenna.
The gap filler signal 114 and the satellite signal 112 have a frequency offset with respect to each other. In the situation where the gap filler signal 114 and the satellite signal 112 are received at the similar strength, a receiving modem may recognize the signals 114 and 112 as interference noises. In this case, the use of the internal antenna only, compared with the use of the external antenna, contributes to an increase in the SNR, improving reception performance of the DMB service.
Presently, however, the user has no way to distinguish the area where the external antenna should be used, from the area where the external antenna should not be used. In most cases, the user suffers deterioration in reception quality as he/she cannot discriminate one case requiring the use of the external antenna from the other case not requiring the use of the external antenna. That is, the user may lose the opportunity to receive higher-quality services with the use of the external antenna. In addition, when the user frequently moves between the two areas, the user cannot normally receive the signals, failing to successfully receive the satellite DMB service.